CF is characterised by abnormal ion transport in several cell types, but most notably in cells which line the airways. This leads to a build-up of thick mucus which sticks to the airways, resulting in a 'remodelling' of the lungs. This increases the prevalence of bacterial and fungal infections in the airways, though the underlying reason for this is still disputed. One theory is that the make-up of the secretions—which are different in CF sufferers compared to the general population—allow bacteria and fungi to evade the immune system with more success.
It's widely accepted that both Candida and Aspergillus species colonise the airways in CF patients, and this is most likely due to long-term exposure to antibiotics. Candida spp. are usually considered relatively benign in most patients, and there is insufficient scientific evidence to treat the presence of Aspergillus species unless it is present and causing allergic bronchopulmonary aspergillosis (ABPA).
Whilst not itself an invasive disease, ABPA exacerbates existing CF disease through wheezing and, potentially, damaging the lung structure through scarring (fibrosis). Unfortunately the diagnosis of ABPA in CF patients is challenging, since many symptoms overlap. Strong laboratory evidence is essential to confirm diagnosis of ABPA. Once diagnosed, the gold standard treatment consists of steroid drugs to help suppress the excessive immune response, alongside itraconazole (an azole antifungal drug) to help prevent fungi taking advantage of any reduced immune response. This halts progression of disease and successfully manages most patient's symptoms.
Whilst Candida and Aspergillus make up the large majority of cultured fungi from CF patients, there are still significant numbers of other pathogenic fungi which infect in CF. Scedosporium and Penicillium species are found in around 15% and 8% of cultures respectively. Similar treatment options exist to those of Aspergillus infections, with laboratory testing again essential to determine which drugs are most effective to the particular organism.